In the chemical-, pharmaceutical industry and in other industrial sectors it is an essential task to produce crystalline, solid material from solution, which is easily treatable in the course of subsequent processes. Several crystallizers are known, functioning with cooling, distillation, evaporation cooling or pulverization.
A cooling crystallizer with grading screw is described by: Dr. Mucskai L. "Kristalyositas" ("Crystallization"), Muszaki Konyvkiado, Budapest, 1971, page 151. This equipment has a mother liquor tank, cooler and grading screw at an angle above the mother liquor tank. The warm, fresh solution passes into the mother liquor tank where it is mixed with the mother liquor arriving on overflow from the grading screw. The crystal slurry after passing through the cooler enters the grading screw, which has a sloping, trough-shaped bottom, on the lower part of which the larger crystals settle. These are removed from the trough by the screw. The mother liquor overflowing from the grader--together with the smaller crystals--flows back into the mother liquor tank, then into the circulation of the cooling cycle. A disadvantage of this process is that the method of grading to two fractions--the material discharged by the screw and the material returned with the mother liquor--is difficult, because the grading has to be ensured by regulating the velocity of the flowing medium, at the same time the crystallization process is also a function of the flow velocity. Owing to these difficulties, this process has not gained acceptance.